1. Field of the Invention
The present invention generally relates to an apparatus and method of recording data on and reproducing data from a recording medium and, more particularly, to recording medium recording and reproducing apparatus and method, by which the distance between an objective lens and an auxiliary lens is arbitrarily changed correspondingly to the thickness of a substrate of a recording medium.
2. Description of the Related Art
In recent years, techniques of recording information or data on a recording medium and of reproducing information or data therefrom by compressing video signals and audio signals at high densities in compliance with MPEG (Moving Picture Expert Group) system or the like have been being put into practical use. In the field of AV (audio visual) techniques, DVD (namely, a digital video disk (or disc)) attracts attention as a next-generation information recording medium of an optical disk unit. Further, in the field of computer technology, an optical disk of the magneto-optical recording type and an optical disk of the phase change type receive attention as such next-generation information recording media. Moreover, it has been proposed that video signals and audio signals are recorded on DVD in compliance with MPEG system and that mass data, which is an extremely large amount of data as never recorded on a single optical disk, is recorded on a recordable optical disk.
Thus, in the case of disk-like optical recording media, such as DVD, which aim at increasing storage capacity to an extremely large capacity, it is necessary to record digital data on the disk at high density to decrease the spot size of a light beam in such a manner as to be smaller than the spot size of a light beam used to record data or information on a conventional recording medium (for instance, CD (namely, Compact Disk)).
Here, let R, .theta. and .lambda. denote the radius of a spot caused by a light beam, an angle of emission of an objective lens and the wavelength of the light beam, respectively. Generally, the radius of a spot of a light beam is given by the following equation (1): EQU R=0.32 .lambda./SIN.theta. (1)
As is understood from the equation (1), it is sufficient for decreasing the (spot) size of the spot caused by the light beam to reduce the wavelength .lambda. of the light beam and/or increase the numerical aperture NA (namely, SIN.theta.(=n*SIN.theta. (incidentally, a refractive index n is 1 in the air)) of the objective lens.
Further, it is sufficient for increasing the numerical aperture NA of the objective lens to increase the diameter of the objective lens. However, if the diameter of the objective lens is increased, not only the size but also the mass of an optical head, which is used for recording information on a disk and for reproducing information therefrom, should be increased. Consequently, it becomes difficult to perform a focus control operation and a tracking control operation.
Thus, there has been proposed a (conventional) system adapted to irradiate a disk with optical beams, which are used for recording or reproducing information, by utilizing a solid immersion lens (namely, what is called a hemispherical lens), as disclosed in U.S. Pat. No. 5,125,750.
In the case of the proposed system, light beams are converged by an objective lens L1 and are then incident on a solid immersion lens L2, which has an incidence surface formed as a spherical surface and further has an emission surface formed as a flat surface, as illustrated in FIG. 6. Incident light coming from the object lens L1 is incident on the solid immersion lens L2 in a direction perpendicular to the spherical surface of the solid immersion lens L2. Thus, the light beams are converged or focused on the center of the flat emission surface of the lens L2.
In the case that the refractive index of the solid immersion lens L2 is n, the numerical aperture is nSIN.theta.. Thus, as compared with the case of a system which is not provided with the solid immersion lens L2, the numerical aperture of the objective lens of the proposed system is substantially n times that of an objective lens of the system which does not employ the solid immersion lens L2. Therefore, a high numerical aperture can be realized without increasing the diameter of an objective lens by using a two-group (namely, a doublet lens) composed of the objective lens L1 and a solid immersion lens L2. Namely, the radius of the spot obtained from the light beam is (1/n) the radius of a spot in the case of using the objective lens singly. Thus, information can be reproduced from a recording medium (namely, a disk) having recording density which is n.sup.2 times the recording density in the case of using the objective lens singly.
Incidentally, actually, there is the necessity of converging light beams, which are emitted from the solid immersion lens L2, on a disk (not shown). Thus, the thickness of the solid immersion lens L2 is set at a value, which is smaller than an original value by the thickness of the disk substrate, so that an actual converging point is located on the disk.
Further, to obtain a further larger numerical aperture NA, it is devised that light beams emitted from the objective lens L1 are used in such a manner as to be somewhat refracted on the spherical surface of the solid immersion lens L2 as illustrated in FIG. 7.
Meanwhile, in the case of applying such a two-group lens to an optical head, for example, it is devised that the objective lens L1 and the solid immersion lens L2 are provided in such a way as to be integral with each other and are mounted on a floating head (namely, a flying head) and that the distance between the floating head and a disk is controlled according to a floating amount (of the disk). The floating amount is, however, controlled according to the linear velocity of the disk. Thus, the floating amount changes according to the linear velocity of the disk. It is, therefore, expected that a change in the distance between the disk and the solid immersion lens L2 causes spherical aberration which acts as a disturbance to reproduction (or reproducing) signals and that thus, it is difficult to obtain favorable reproduction signals.
Moreover, there are variations in the thickness of each of a light-transmissible substrate and a solid immersion lens, which compose a disk. The variations cause the spherical aberration. In this case, the conventional system has a problem that it is difficult to record information on or reproduce information from a recording medium.
The present invention is accomplished in view of such a situation or problem of the prior art.